docs(user): add a model-provider configuration guide

The guide tier said how to compose plugins with `cordis.yml` but never how to
reach a provider other than DeepSeek, so the two things a person actually does
— give a catalog provider its key from the Models page, and declare a gateway
the installed catalog does not ship — had no home outside package READMEs.

The new page covers both entry points and the relationship between them: the
Models page and `$DSH_HOME/settings.yaml` write one document, over a
`llm-pi-ai` adapter that mounts dormant until that document names routes. It
carries the settings shape, catalog replacement and its capacity fallbacks,
credential references, and the four failures a misconfigured route produces,
and links the generated config catalog for exhaustive fields.

It sits between Quick start and Configuration in the guide sidebar, which is
where a reader hits the question.

docs/user/guide/quickstart.zh.md

@@ -57,5 +57,6 @@ headless-agent 使用 `@deepseek-ai/dsh-cli-demo` app。`dsh web` 则组合 [`ap
 
 ## 下一步
 
+- [配置模型提供方](./providers.md) — 接入 DeepSeek 之外的提供方与自定义网关
 - [配置文件](./config.md) — 了解 `cordis.yml` 的格式
 - [开发插件](../develop/basic/) — 编写自己的 tool 或后端